Abstract

While telomerase-mediated DNA extension is the most commonly used
mode of eukaryotic chromosome end maintenance, the requirement of
stable chromosome ends for genomic stability has selected for the
evolution of alternative strategies. Telomerase deletion leads to progressive
loss of telomeric DNA and loss of viability for the majority of cells.
However, populations of survivors arise. In fission yeast, these survivors
either maintain their telomeres via recombination or survive complete
telomere loss by undergoing intra-chromosomal fusion (circularisation) of
their chromosomes. Previous work in the lab led to the identification of a
novel class of survivors that appear to lack telomeric DNA, but do not
circularise their chromosomes. Intriguingly, these novel survivors have
amplified either of two classes of repetitive sequences at their chromosome
ends, the rDNA or the subtelomeric elements; in the latter case, these
elements have also spread throughout the genome. This thesis involves the
characterisation of these survivors, named HAATI survivors. The amplified
repetitive sequences in HAATI are associated with heterochromatin and we
find that the heterochromatin machinery is crucial for HAATI survival.
Furthermore, chromosome linearity in HAATI relies on Pot1, a canonical
chromosome end protection factor whose recruitment to chromosome ends
had been thought to rely on telomere sequences. Our data suggest that
Pot1 localises to HAATI chromosome ends via interactions with the
heterochromatin machinery and non-telomeric single strand overhangs at
the chromosome termini. This discovery not only reveals a previously
unrecognised mode by which cancer cells could escape the requirement
for telomerase activation, but also provides a tool for studying genomes that
sustain unusually high levels of heterochromatinization.